Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGby the action of the piston. The timing of thevalves in a supercharged 4-stroke cycle engineis also different from that in a similar enginewhich is not supercharged. In the superchargedengine the intake-valve opening is advanced andthe exhaust-valve closing is retarded so thatthere is considerable overlap of the intake andexhaust events. This overlap increases power,the amount of the increase depending upon thesupercharging pressure. The increased overlapof the valve openings in a supercharged 4-stroke cycle engine also permits the air pressurecreated by the blower to be used in removinggases from the cylinder during the exhaustevent. How the opening and the closing of theintake and exhaust valves or ports affect bothscavenging and supercharging, and the differencesin these processes as they occur insupercharged 2- and 4-stroke cycle engines,can be seen by studying the diagrams in figure22-22.As in the case of the diagrams used in connectionwith the discussion of engine operatingprinciples, the circular pattern in figure 22-22represents crankshaft rotation. Some of theevents occurring in the cycles are shown interms of degrees of shaft rotation. However,the numbers (of degrees) shown on the diagramsare for purposes of illustration and comparisononly. When these diagrams are being studied,it must be kept in mind that the crankshaft of a4-stroke cycle engine makes two complete revolutionsin one cycle of operation while the shaftin a 2-stroke cycle engine makes only one revolutionper cycle. It should also be rememberedthat the exhaust and intake events in a 2-strokecycle engine do not involve complete pistonstrokes as they do in a 4-stroke cycle engine.Even though the primary purpose of a dieselengine intake system is to supply the air requiredfor combustion, the system generallyhas to perform one or more additional functions.In most cases, the system cleans the air andreduces the noise created by the air as it entersthe engine. In order to accomplish the functionsof intake, an intake system may include an airsilencer, an air cleaner and screen, an air boxor header, intake valves or ports, a blower, anair heater, and an air cooler. All of these partsare not common to every intake system. Anintake system in which only a silencer, a screen,a blower, an air box, and intake ports providea clean supply of air, with minimum noise, tothe combustion spaces is shown in figure 22-20.FOURSTROKECYCLETWOSTROKECYCLET.D.C.B.D.C.54.19:.20BFigure 22-22.—Scavenging and supercharging indiesel engines.The system which functions primarily to conveygases away from the cylinders of an engineis called the exhaust system . In addition to thisprincipal function, an exhaust system may bedesigned to perform one or more of the followingfunctions: muffle exhaust noise, quenchsparks, remove solid material from exhaustgases, and furnish energy to a turbine-drivensupercharger. The principal parts which maybe used in combination to accomplish the functionsof an engine exhaust system are shown infigures 22-20 and 22-23.568